1. Field of the Invention
The present invention relates to an exposure calculation device for a camera which performs photometry on the photographic field and calculates an exposure value with which errors in photometry due to noise and the like are prevented.
2. Description of the Related Art
Recently more and more cameras have been produced which perform a so called divided photometry method. With this divided photometry method, photometry is performed on a plurality of regions into which the photographic field is divided, and a photometric signal according to a brightness for each of these regions is obtained. For this method, the photometric element (a photoelectric conversion element) is divided into a plurality of sub-elements which correspond to the above described plurality of regions on the photographic field. Further, an exposure value is calculated based upon the photometric signals for the various regions, and a shutter time and aperture opening value are determined according to this exposure value, so that photography of the principal object to be photographed can be performed with appropriate exposure settings, without the illumination level of the background exerting any effect.
With this type of divided photometry method, the greater is the number of sub-elements into which the photometric element is subdivided, the greater is the resolution which can be obtained for the distribution of illumination over the principal object to be photographed and the area surrounding it, and accordingly the more appropriate exposure value can be calculated. Therefore in recent years this number of sub-elements has increased, and in the near future it is expected that in some devices the number of sub-elements will be in the range between several tens and several hundreds.
The output from each of the divided sub-elements of the photometric element needs to be amplified by an amplifier for input to a processing circuit, but when the number of sub-elements is in the range of several tens, or even worse, several hundreds, it is a practical impossibility to provide an amplifier for each of the sub-elements, respectively. Therefore in this connection a construction has been conceived of, when for example a charge coupled device (a CCD) which comprises a number of photoelectric conversion sub-elements arranged in a two dimensional array is used as the photometric element, in which the outputs of all the sub-elements are input in series to a single amplifier.
However, when this type of charge coupled device is used, a so called dark current component is present within the photometric signal, and the problem arises that this can cause errors in the photometric process. This dark current is a type of noise component engendered by the construction of the electrical circuitry associated with each sub-element. In particular the relative proportion of this noise component included in the photometric signal becomes great when the value of the photometric signal is relatively small (i.e. when an object to be photographed is relatively dark), so that at this time the errors in the photometric process become large. However with prior art cameras there has been no function for suppression of the noise component included in the photometric signal, and for this reason the problem has arisen that it is not possible to calculate an accurate exposure value. In particular, the higher is the ambient temperature the greater becomes the value of the above described dark current, so that at high temperatures the possibility of performing photography with an exposure value which is inappropriate becomes high.
Further, when a divided type photometric element is made up from silicon photodiodes (SPDs), in conditions of even illumination the output currents generated in the various sub-elements are not necessarily the same, because the sensitivity characteristics typically vary between individual ones of the diodes. For this reason, in order to determine appropriate exposure conditions by calculation based upon the photometric signals output from the various photometric sub-elements, in a ROM or the like a plurality of compensation values corresponding to these various photometric sub-elements are stored so that the non-uniformity of their sensitivity characteristics can be eliminated, and then an exposure amount is calculated after photometric values have been compensated based upon these compensation values. For example, in case that the photometric element is made up from five separate SPDs, if a compensation value of eight bits is provided for each of these separate SPDs, then for storage of these five compensation values it will be sufficient to provide a storing circuit (e.g. a non volatile memory) with a maximum capacity of forty bits.
However, if as described above a photometric element is used which is divided into several tens of sub-elements, since several tens of compensation values are required, a ROM of capacity of at least several hundred bits is required, and accordingly problems arise such as that the size of the package increases and also the cost becomes high. Further, when as is contemplated in the near future the number of sub-elements included in the photometric element comes to be in the range of several hundreds, these problems will become more and more troublesome.
Further, when with the above described type of divided photometry method silicon diodes are used as the photometric element, if light of excessive strength falls upon part of the region of the photometric element, the current generated in this partial region has a tendency to leak over to other neighboring sub-elements of the photometric element, and the problem arises of errors occurring in their proper output. Electric potential barriers are provided between the various regions of the photometric element, in order that electric charge generated in any one region should not leak over to the neighboring regions. However, errors can occur in the detected electric current if the electric charge generated in a partial region of the photometric element on which light of excessive strength has fallen manages to jump over one of these electric potential barriers and leaks to a neighboring region.
The above described type of phenomenon is generally called blooming or smearing according to the circumstances in which it is generated and the like. In such cases the output signal which shows the actual illumination on the portion of the photometric element on which light of excessive strength has fallen or on its neighboring region is corrupted. With a two dimensional sensor of the type used in a video camera or the like, even if this type of blooming occurs, it is possible easily to detect it by observing the monitor image. Therefore it is possible to institute countermeasures such as changing the lens aperture opening value or the like.
However, with a camera for the normal type of film photography, calculations are performed based upon the signals output by the photometric element, and an exposure control value such as the shutter time value or the aperture opening value is only displayed on a display means as a result. Due to this, the problem arises that, even if errors attributable to blooming or the like are present in the signals which are used as inputs for the process of calculation, this cannot be determined until the film is developed.